Author: Jefferson H. Dickey MD

Introduction

In 1996, many of the academic researchers published reviews of the health effects of air pollution in the peer reviewed medical literature. This was mostly in anticipation of the legislature and court mandated EPA review of the ozone and particulate National Ambient Air Quality Standards (NAAQS). Most of these reviews recognized the substantial association in laboratory and epidemiologic studies between air pollutants and adverse health outcomes. The EPA agreed, and promulgated new NAAQS for ozone and particulate. A few researchers felt the case for particulate to have already been shown to be causal.1 While the evidence was substantial, coherent, consistent, robust, and almost overwhelming, a minority of researchers dissented.2 Many researchers found the associations compelling but felt the need to ask additional questions.3 Similarly, regarding ozone, few doubted the evidence that it could cause adverse effects, the question was whether the magnitude of the effects were sufficient to be considered of public health import. Substantial recent data is finding associations between ozone levels and acute and chronic declines in lung function, onset of chronic lung disease, and daily mortality rates.

This review is limited to the criteria air pollutants which would be expected to result from power plant emissions. This report does not address data on pollutants which are unique to motor vehicles (diesel exhaust), nor does it address mercury, which is emitted by power plants and is increasingly being recognized as causing potentially serious environmental contamination.

Hence, this review addresses NOx, SO2, ozone, and particulate (PM) air pollution. Not every study done in the last 4 years is addressed; however, a substantial effort is made to include every study which has implications for considering the health effects of these pollutants in the general population. I begin with a review of the major pollutants, their sources, a sense of the levels to which the general population has been exposed, and a summary of the health literature developed up until 1996.

Then, the recent health effects literature is summarized with emphasis on the data developed since 1996. The recent literature is divided into 15 sections according to the general type of study design. This analysis finds that within each study type, substantial data has emerged which supports the notion that air pollution of the types derived from power plant emissions is associated with clinically significant impairment public health. Taken as a whole, the coherence of the data inexorably leads to the suggestion that these air pollutants are causing substantial adverse health outcomes.

In February 2000, Environmental Health Perspectives, a scientific journal published by the National Institutes of Health environmental division, published an editorial stating that the evidence on particulate air pollution was now sufficient to be considered causal. With considerations of unnecessarily high air pollutant output from outdated power plants, and substantial new research on health effects of those pollutants, the time seemed appropriate to review the recent data and facilitate its public dissemination.

Information on local and regional air pollution levels and major sources is usually available on the US EPA website (www.epa.gov).

While visibility has improved in the west but perhaps worsened in the east, improvement has been seen with many measures of air pollution over the last couple decades (60% lower sulfur dioxide and carbon monoxide levels); however, in 1996 roughly 46 million people in the USA still lived in areas not meeting the US EPA National Ambient Air Quality Standards (NAAQS), remaining at risk for adverse health consequences.

Sulfur Dioxide (SO2)

Sulfur Dioxide (SO2) gas is formed during the combustion of sulfur-containing fossil fuel (coal and oil), during metal smelting, paper manufacturing, food preparation, and other industrial processes. It is an important contributor to acid aerosols and "acid rain", and is typically a component of complex pollutant mixtures. Peak one hour SO2 values recently reported by the EPA occur in the 0.4 to 0.8 ppm range, with rare higher excursions. Relatively cheap high-sulfur coal is most extensively used by power plants in the U.S. midwest and east, leading to downwind acidification of lakes in New England and eastern Canada.

Nitrogen Dioxide (NO2)

Fossil fuel combustion generates nitrogen dioxide (NO2) and nitric oxide (NO) which is rapidly oxidized to NO2. NO2 reacts in the presence of sunlight and VOCs to form ozone, and contributes through atmospheric reactions to the formation of nitrous and nitric acid aerosols. Hence, in epidemiologic studies differentiation of individual pollutant effects is very difficult and often impossible. Major sources are motor vehicles, power plants, and other fossil fuel burning industries. Local levels tend to vary with traffic density. Indoor exposures to NO2 can be substantial from unvented combustion sources, such as gas stoves, and space heaters. In the absence of indoor sources, indoor levels are about half of those outdoors. Individual NO2 exposure is correlated rather poorly with the fixed site ambient air NO2 levels at least partially because of the high proportion of time spent indoors. The highest ambient one hour exposures reported by EPA are over 0.200 ppm, and the highest annual mean exposures are over 0.040 ppm.

Ozone (O3)

Ozone has different health implications in the stratosphere and the troposphere. In the stratosphere (the "ozone layer"), 10-50 km (6-30 miles) above the earth, ozone provides a critical barrier to solar ultraviolet radiation, and protection from skin cancers, cataracts, and serious ecological disruption. International treaties phasing out ozone-depleting chemicals like chlorofluorocarbons have eased, but not eliminated the threat to this layer's integrity. Stratospheric ozone is good, tropospheric ozone is bad.

We focus on tropospheric (ground level) ozone pollution. Excessive ozone exposure is widespread: over 70 million people lived in areas not meeting the EPA ozone standard in 1995; that number will increase markedly with implementation of the updated 1997 standard. The highest recent domestic ozone levels have occurred in southern California and Texas, with peak levels in the mid to high 200 ppb range.

Ambient ozone concentrations rise as a result of a solar UV irradiation driving a complex series of reactions involving volatile organic compounds (VOCs) and nitrogen oxides (NOx). Community sources of VOCs include gasoline vapors, chemical solvents, combustion products of fuels, terpenes, and consumer products, while NOx is largely generated by fossil fuel combustion (power plants, diesel, and industrial boilers). Ozone levels may paradoxically be as high or higher downwind from cities than in the cities themselves, reflecting the time needed for the atmospheric photochemical reactions to occur. Similarly, when air masses are stagnant for a few days, precursor concentrations rise and react in the sunlight resulting in exceptionally high levels. This is most likely where large urban areas are surrounded by mountains, such as Los Angeles and Mexico City, but significantly elevated ozone levels occur sporadically in many areas of the U.S.

Particulate

About 24 million persons in the U.S. lived in areas not meeting the U.S. EPA's National Ambient Air Quality Standard (NAAQS) for airborne particles less than 10 micrometers in aerodynamic diameter (PM10) in 1995.

Particulate air pollution (PM: particulate matter) is a heterogeneous classification of liquid and solid aerosols which includes anthropogenic emissions from power plants and other fuel combustion (coal, oil, biomass), transportation, and high temperature industrial processes. Smaller particles (often less than PM3) include viruses and some bacteria, but mostly come from anthropogenic sources, including sulfate and nitrate aerosols and other combustion derived atmospheric reaction products; whereas larger particles (PM3 to 30) include pollen, spores, crustal dusts, and other mechanically generated dusts. Size is a critical determinant of deposition site, with larger particles (greater than PM3) tending to deposit in the nasal and tracheobronchial regions), and smaller particles (less than PM10) penetrating deeper into the lungs. Regional air pollution exacerbations may be due to intermediate and long range tropospheric transport in addition to local influences such strong emission sources, proximity to heavily used roadways, stagnant air masses, and local weather temperature inversions. Recent EPA data shows typical peak community levels in the low 200s to mid 300s, with rare exposures as high as 700 mcg/m3. Contributing species include sulfur and nitrogen oxides (forming sulfuric, nitric, and nitrous acids), metals, ammonium salts, mechanically generated dusts (silica, etc), some with adherent polycyclic aromatic hydrocarbons, dioxins, dibenzofurans, etc, and is usually present as a complex mixture with atmospheric reaction byproducts. Acid aerosols refers to particulate which consists of acidic chemicals (including hydrated sulfur and nitrogen oxides).